Luminescence imaging of living animals can be shown to be a useful technique for obtaining information on both normal and diseased biological processes taking place in that particular animal. The images can generally be obtained with low light level charge-coupled device (“CCD”) cameras or other sensitive optical imaging devices, and the biological process can be visualized using luminescent reporters which are produced by or which target some compound (e.g., an enzyme, protein, etc.), or some cell surface receptor which can be part of the process. The images generally show regions of optical emission which can be correlated with the biological process. Sources of visible light emission can either be nuclear reporter probes as a source for Cerenkov light or a luciferase-luciferin reporter system as a source for bioluminescent light.
A source of light emission can be provided by a Cerenkov luminescence reporter system, in which Cerenkov radiation can be emitted when a charged particle (e.g., an electron or positron), passes through a medium, such as biological tissue, with a speed greater than the speed of light in the same medium. The charged particle can disrupt the local electromagnetic field of the medium by displacing the electrons of the atoms. Radiation can be emitted when the displaced electrons fall back into an equilibrium state. Beta minus (i.e., electrons) or beta plus (i.e., positrons) radiation above the threshold velocity (i.e., energy>200 keV), which is generated by the decay of nuclear reporter probes in biological tissue, can produce Cerenkov radiation at wavelengths of visible light. The light can be detected with a sensitive optical camera and, hence, functions as an optical reporter signal for beta minus/plus nuclear reporter probes (e.g., fluoro-D-deoxyglucose) in nuclear imaging.
Further, the Cerenkov light in the visible and near-infrared is typically strongly scattered in biological tissue. Therefore, planar or two-dimensional (2D) images of Cerenkov radiation at the tissue surface obtained with current imaging technology typically contain little information about the actual depth and strength of the light emitting beta radiation, i.e., nuclear reporter probe. Hence, tomographic imaging technology is generally needed to retrieve three-dimensional (3D) information about the “true” reporter probe distribution and source of Cerenkov radiation. Such technology can facilitate quantitative imaging of nuclear reporter probes via Cerenkov radiation.
A source of light emission can also be provided by a bioluminescent luciferase/luciferin reporter system, in which a target of interest can be transfected with a luc gene that expresses the enzyme luciferase at the target site. A substrate, luciferin, can be administered to the animal and can be distributed throughout the animal tissue. The enzyme luciferase can catalyze a chemoluminescent reaction of luciferin at the target site, which can result in light emission with a range of wavelengths between 500 and 700 nm. Light at these wavelengths, however, is typically multiply scattered in the tissue and, thus, diffuse light distributions on the tissue surface can be measured with 2D planar imaging techniques. Hence, no direct image of the bioluminescent reporter probe's location or emission strength can typically be obtained. Moreover, light absorption by tissue chromophores, such as (oxy-)hemoglobin, significantly attenuate the optical signal. Therefore, without tomographic reconstruction, which can take the scattering and absorption effects into account, accurate determination of absolute light emission levels is generally not feasible.
Luminescence tomography has the potential to overcome the limitations of planar luminescence imaging and can provide precise spatial location and emission strength of light emission levels of reporter probes within the tissue. In luminescence tomography, 2D luminescence images can be used together with a light propagation model and a reconstruction procedure that retrieves the 3D source distribution. The light propagation model, also termed forward problem, can be generally based on partial-differential equations (PDE) for the photon flux inside the tissue. These PDEs can be solved with numerical techniques, which can calculate the boundary current of light on the tissue surface for a given set of optical parameters and source points inside the tissue. The image reconstruction procedure can solve the inverse source problem determining the source location and strength inside the tissue domain given the planar luminescence images taken on the domain boundary. The inverse problem can be both highly ill-posed due to strong light scattering and can be underdetermined due to the limited boundary measurement data. Moreover, there can be errors in the modeled light propagation solution owing to the uncertainty in the optical tissue properties, which are known to vary in their spectral optical properties. These properties are often determined from ex vivo measurements that may not be representative of the in vivo conditions.
Accordingly, it may be beneficial to address at least some of the issues and/or problems described herein above.